1. Field of the Invention
The present invention pertains to a method and apparatus for molding lenses, and particularly to a method and apparatus for molding lenses such as contact lenses which have a finished edge and which are suitable for wearing directly on the eye.
2. Description of the Related Art
It has become desirable to form lenses, particularly contact lenses but also other types of lenses, by molding such lenses rather than by lathing or by other processes. Molded lenses are desirable for several reasons. For example, it is possible to produce such lenses with great repeatability--to produce many lenses which have the same shape and optical characteristics. Moreover, a molded lens can be formed to any desired shape, subject only to producibility constraints of the mold.
In general, molded lenses are formed by depositing a curable liquid such as a polymerizable monomer into a mold cavity, curing the liquid into a solid state, opening the mold cavity and removing the lens. Other processing steps, for example, hydration, may also be performed. In any event, as the lens material transitions from its liquid or semi-liquid state to a solid or semi-solid state, the material shrinks. For example, when the lens material is a polymerizable monomer such as hydroxyethyl methacrylate ("HEMA"), about 15% to 25% volume reduction can be expected as the material cures.
Material shrinkage is of considerable concern and must be accommodated during molding. If shrinkage is not properly accommodated, it is possible for the curing lens material to pull away from its associated mold surface. Any such separation produces unacceptable optical surfaces and results in an unusable lens. And even if separation does not occur, internal stresses often produce unacceptable distortion of the lens.
Because of the problem of lens material shrinkage during curing, it has heretofore been difficult to provide acceptable molded lenses that do not require additional finishing steps. Conventional molding techniques, such as described in Larsen U.S. Pat. No. 4,565,348, accommodate shrinkage with mold halves which flex during curing. Such techniques may have unacceptable repeatability due to unpredictable deformation in mold shape during curing. In particular, because the lens shape is defined by a mold half that flexes during curing, the radii that define the optical power of the lens can change unpredictably making it difficult to produce lenses repeatably.
The geometry of contact lenses makes the problems associated with shrinkage even more acute. FIG. 1 is cross-sectional view of a contact lens 11 which, as shown there, includes a central optical zone 12 and a peripheral carrier zone 14. Typically, the optical zone 12 is 7 mm-11 mn in diameter and the overall diameter of the lens 11 is 13 mm-15 mm. As seen in FIG. 1, the lens is formed in distinct radius zones on both the posterior side 15 and the anterior side 16. Thus, from the central optical zone 12 outward, anterior side 16 is formed first with radius R1, which is selected in accordance with desired optical power of the lens, merges into radius R2 in the carrier zone 14 of the lens, and then merges into radius R3 selected to provide a suitable transition to the edge taper. On the posterior side 15, the optical zone 12 is formed with radius R4 which is also selected in accordance with the desired optical power of the lens, and which merges with radius R5 in the carrier zone 14 of the lens.
In consequence of this configuration, lens 11 is formed with substantially greater volume of material in the peripheral carrier zone 14 than in the central optical zone 12. That is, whether the optical zone 12 provides positive or negative optical power, because carrier zone 14 surrounds optical zone 12 at a greater diameter, there is more material in the carrier zone. Consequently, there is significantly more shrinkage that must be accommodated in the peripheral portion of the lens than there is in the central portion thereof, and the lens material therefore shrinks nonuniformly.
Conventional molding techniques, such as described in the above-mentioned Larsen U.S. Pat. No. 4,565,348, do not compensate satisfactorily for this nonuniform shrinkage. The flexible molds provide their greatest compensation in the middle of the mold cavity and provide no compensation at the periphery of the mold where the greatest shrinkage, as described above, occurs.
It has also been considered to accommodate shrinkage by providing a reservoir of suitable polymerizable monomer at the peripheral region of the mold cavity. See U.S. Pat. Nos. 4,113,224 and 4,197,266. In principle, during polymerization, the shrinking monomer will draw additional monomer in from the reservoir. In practice, this configuration has not been found satisfactory inasmuch as it is difficult to mold a finished edge into the lens, and such a lens therefore inevitably requires subsequent machining and lathing processes. Moreover, because the most shrinkage occurs after the lens material gels, reservoir techniques are of limited effectiveness.